Method of automated demodulation and classification of phase-shift-keying signals using hysteretic differential zero-crossing time samples

ABSTRACT

An automatic zero-crossing signal demodulation and classification device for rapidly identifying unknown modulation in a signal identifies unknown modulation in a signal, demodulates differential phase shift keying signals and automatically recognizes certain phase shift keying signals. This is accomplished by eliminating unknown term f c  in differential phase estimation, introducing a symbol rate tracking mechanism, applying hysteresis nonlinearity to eliminate phase shaping effect and using weighted average to estimate phase difference. Better estimates are accomplished by using hysteretic nonlinear function to detect zero-crossing points in eliminating false detecting of zero-crossing points caused by additive noise, and calculating differential phase without directly using center frequency to simplify estimation process. Present invention also encompasses automated zero-crossing signal surveillance demodulation and classification device for rapidly identifying unknown modulation in a signal and method for automatic zero-crossing demodulation and classification of unknown modulation signal.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, and licensedby or for the United States Government for governmental purposes withoutpayment to me of any royalties thereon.

FIELD OF THE INVENTION

The present invention relates generally to the field of phase-shiftkeying signals and more particularly to the field of automatedmodulation classification of phase-shift-keying signals with azero-crossing time sample technique.

BACKGROUND OF THE INVENTION

Automated modulation classification of signals is an extremely usefultechnique for both military and commercial communications equipment. Innon-cooperative communications such as signal surveillance and somecognitive radio applications, the modulation scheme is unknown and hasto be estimated and classified automatically. Continuing research anddevelopment has led to steady progress and advances in automatedmodulation classification techniques over the years. However, thesetechniques still suffer from a number of difficulties and limitationswhen being implemented in non-cooperative environments in the field,because of unknown parameters such as signal and noise power, carrierfrequency and pulse shape, and so on. Those skilled in the art recognizethat prior art signal classification equipment, techniques and methodsneed to be more robust in order to perform adequately in harshenvironmental conditions.

One potentially promising technique is the zero-crossing approach, whichshould be relatively simple to implement, but to date has not yet beensuccessfully accomplished. Recognizing phase-shift-keying (PSK) with azero-crossing approach has only met with limited success.

A brief examination of the zero-crossing approach in signal repetitionrate estimation points to a few of the most noteworthy difficulties withthat approach. If we denote f(k), k=1, 2, . . . , as a digitizedintermediate frequency (IF) copy of an unknown phase-shifted keyingsignal at time t(k), then denote a subset of t(k) as x(1), x(2), . . .which are zero-crossing samples of f(k), i.e. f(x(i))=0 for all i, andalso denote φ(1), φ(2), . . . as phase symbols of f(k), then one canestimate the relative phase of f(k) at the m^(th) symbol according tothis expression:

$\begin{matrix}{{\phi(m)} = {2\pi\; f_{c}{\left\{ {\frac{1}{L_{m}}{\sum\limits_{n = j_{m}}^{j_{m} + L_{m} - 1}\left\lbrack {{x(n)} - \frac{n}{2f_{c}}} \right\rbrack}} \right\}.}}} & {{Equation}\mspace{14mu}(1)}\end{matrix}$where f_(c) is the center frequency of f(k), L_(m) is the number ofzero-crossing samples within the m^(th) symbol period, and x(n),n=j_(m), j_(m)+1, . . . , j_(m)+L_(m)−1 are zero-crossing points withinthe m^(th) symbol time-period. The underlying assumptions are that thesymbol rate of f(k) is known, the symbol has a square pulse shape, thesymbol timing is perfectly matched and that the center frequency iseither previously known or can be estimated accurately. The differentialphase of f(k), which is denoted by θ(m), is calculated by:θ(m)=[φ(m)−φ(m−1)] mod(2π)  Equation (2)Then, the phase-shifted keying signal f(k) is classified by correlatingthe histogram of θ(2), θ(3), . . . , θ(M) with a number of knowntemplates in order to determine the best match.

This prior art approach suffers from a number of drawbacks. Thedrawbacks include the need to know the center frequency f_(c) accuratelyin Equation 1 in order to estimate φ(m) and the need to conduct symbolestimation and symbol timing precisely in order to align the startingpoint x(j) and ending point x(j+L_(m)−1) within the desired symboltime-period in order to reliably detect zero-crossing points. Anotherdrawback is the lack of reliability in detecting zero-crossing pointsdue to the additive noise. If the zero-crossing points x(i)=t(a) are notdetected due to noise, but points x(i+1)=t(b) are still detected inspite of the noise, then x(i)=t(b) will be mistakenly used in Equation 1and consequently all further phase estimates after time t(a) will alsobe incorrect. The prior art approach also requires a square pulse shape,and if a square pulse shape is not available, then the zero-crossingpoints near the pulse edges will be dominated by noise and cause faultydetections. These kinds of limitations of the zero-crossing approach forsignal repetition rate estimation devices and techniques along withlong-standing prior art difficulties in modulated classification such asquestionable strength in the face of harsh environmental conditions havecreated a long-felt need for a zero-crossing point estimating techniquethat is faster, more robust and more accurate than current zero-crossingmodulation classification techniques. Up until now, there is noavailable zero-crossing demodulation and classification approach thatovercomes the long-standing limitations, shortcomings and disadvantagesof the prior art equipment and techniques.

SUMMARY OF THE INVENTION

In order to overcome the long-standing prior art drawbacks caused byneed to solve the unknown center frequency, f_(c), achieve perfect pulsetiming estimation, the need for a square pulse shape, and the excessivesensitivity to noise in detecting zero-crossing points, the presentinvention provides automatic zero-crossing signal demodulation andclassification device for rapidly identifying an unknown modulation in asignal. Despite the fact that the zero-crossing technique would berelatively simple to implement in classifying signals and is not abaseband constellation based system, the zero-crossing approach has notyet been successfully used for classifying signals.

Zero-crossing, or level crossing, time sampling, has often been used innumerous electronic applications such as frequency estimation,frequency-drift estimation, angular velocity estimation and signalmodulation classification. The term “zero-crossing point” refers to thepoint where a periodic waveform varies from a positive value to anegative one and crosses the zero value in the process. This means thata waveform with only a positive or a negative value has no zero-crossingpoint, but if it varies up or down it may cross a given non-zero level,which is also known as the level-crossing point. Zero-crossing timeestimation takes the average of the time differences between twozero-crossing points of a periodic function.

The present invention fulfills the long-felt need for a more rapid,robust and accurate estimating technique than prior art zero-crossingestimation techniques by providing automated demodulation andclassification zero-crossing estimation devices and methods. Thisinvention overcomes prior art problems by eliminating the unknown termf_(c) in differential phase estimation, introducing a symbol ratetracking mechanism, applying hysteresis nonlinearity to eliminate thephase shaping effect and using a weighted average to estimate the phasedifference.

It is an object of this invention to provide an automated demodulationand classification zero-estimating device for unknown modulationsignals.

It is a further object of this invention to provide an automaticzero-crossing signal demodulation and classification device for rapidlyidentifying an unknown modulation in a signal that classifies anddemodulates differential phase shift keying signals and automaticallyrecognizes certain phase shift keying signals.

It is yet a further object of this invention to provide methods forautomatically classifying the zero-crossing point by demodulatingdifferential phase shift keying signals and automatically recognizingcertain phase shift keying signals.

These and other objects are advantageously accomplished with the presentinvention providing an automated demodulation and classificationzero-estimating device comprising a sample delay, differential phaseestimator, symbol timing circuit, phase histogram, histogram templatesand PSK modulation. Better estimates are accomplished by using thehysteretic nonlinear function to detect the zero-crossing points ineliminating the false detecting of the zero-crossing points caused bythe additive noise, and calculating differential phase without directlyusing the center frequency to simplify the estimation process.

This invention's automated demodulation and classificationzero-estimating approach demodulates M-ary differential phase shiftkeying (M-DPSK) signals and automatically recognizes M-DPSK and M-aryphase shift keying (M-PSK) signals. More rapid, robust and accurateestimates are achieved because the innovative techniques of the presentinvention eliminate the need to accurately estimate the centerfrequency, pulse shape and pulse timing, and they are therefore simplerand more rapid than prior art approaches. This invention's devices andmethods advantageously optimize and improve the user's ability toestimate and classify unknown modulation schemes, which the prior arthas not yet achieved with non-cooperative communications. The presentinvention also encompasses an automated zero-crossing signalsurveillance demodulation and classification device for rapidlyidentifying an unknown modulation in a signal and a method for automaticzero-crossing demodulation and classification of an unknown modulationsignal

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual block diagram of the automated demodulationdevices of the present invention;

FIG. 2 illustrates an example of symbol timing;

FIG. 3 illustrates a timing signal g_(s)(k);

FIG. 4 depicts a function of the relay nonlinear converter in accordancewith the present invention;

FIG. 5 depicts a function of the hysteresis nonlinear converter inaccordance with the present invention;

FIG. 6 is a conceptual diagram of an automated demodulationclassification device in accordance with the present invention;

FIG. 7 is a block diagram for estimating the average number ofzero-crossings in accordance with the present invention;

FIG. 8 is a chart defining the symbol for the zero-crossing counter inaccordance with the present invention;

FIG. 9 is a chart defining the symbol for the trigger logic circuit inaccordance with the present invention;

FIG. 10 is a software flow diagram depicting the operation of theautomated demodulation classification software and methods of thepresent invention;

FIG. 11 is a chart illustrating an example of a QPSK signal inaccordance with the present invention; and

Table 1 is the truth table for the trigger logic in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE DRAWINGS

The automated demodulation estimating device of the present inventionovercomes the problems, disadvantages and shortcomings of the prior artby eliminating the unknown term f_(c) in differential phase estimation,introducing a symbol rate tracking mechanism, applying hysteresisnonlinearity to eliminate the additive noise effect and using a weightedaverage to estimate the phase difference in automated demodulationdevices and methods. In accordance with the present invention, thedigitized and band-pass filtered intermediate frequency (IF) signal f(k)11 is defined as:

$\begin{matrix}{{f(k)} = {{Re}\left\{ {{A_{s}{\mathbb{e}}^{j{({\frac{2{\pi f}_{c}k}{v_{s}} + \theta})}}{\sum\limits_{m}{{\phi(m)}{p\left( {\frac{k}{v_{s}} - \frac{m}{f_{b}}} \right)}}}} + {n(k)}} \right\}}} & {{Equation}\mspace{14mu}(3)}\end{matrix}$where A_(s) is the amplitude, θ is the phase offset, φ(m) represents thesymbol transmitted within the m^(th) period, p(t) is the pulse shapingfunction and n(k) is the additive noise. The symbol sequence {φ(m)} isan independently identically distributed sequence, with its values drawnfrom a finite alphabet specific to each modulation format, where k=1, 2,. . . are the time indices, v_(s) is the given sampling frequency andf_(b) is the symbol frequency which may be known, or can be estimatedusing existing techniques. Accordingly, A_(s) and θ need not be knownand f_(c) does not need to be known precisely. In order to insureaccuracy and resolution in the estimation techniques of the presentinvention, over-sampling is necessary; therefore one must assume thatf_(c)>4 f_(b) and that set f_(s)>4 Mf_(c) for an M-PSK modulated signal.

FIG. 1 is a conceptual block diagram of the automated demodulationdevices of the present invention where the normalized differential phaseof the digitized IF signal is calculated by the differential phaseestimator and a symbol timing circuit is used to control the estimationprocess. In accordance with the present invention, when the symbol iscoded by differential phase-shifted keying with a means for differentialphase shift keying, then the symbol will be determined directly by thedifferential phase estimator. In either case, a histogram of thedifferential phase is formed to show the statistical patterns of thephase-shifted keying (PSK) or differential phase-shifted keying (DPSK).The automated modulation recognition of the present invention isaccomplished by correlating the histogram with the known templates inorder to find the best match.

Referring now to the drawings, FIG. 1 is a conceptual block diagramdepicting a basic embodiment of the automatic zero-crossing signaldemodulation and classification device for rapidly identifying anunknown modulation in a signal 10 of the present invention, comprising anormalized digitized IF signal f(k) 11 that is simultaneously sent to asample delay circuit 12 and symbol timing circuit 13. The delayed(lower) branch of the IF signal f(k) is fed to a differential phaseestimator 14. The symbol timing circuit 13 controls symbol timing andthe overall estimation process. The symbol timing circuit 13 sends asymbol timing input to the differential phase estimator 14. Thedifferential phase estimator 14 demodulates the signals by DPSK with thedifferential phase-shift-keying means and provides a differential phaseoutput 15. If the symbol is coded by DPSK, then the symbol will bedetermined directly by the differential phase estimator 14. An estimatedphased histogram 16 is formed by a means for generating an estimatedphase histogram to show the statistical patterns of the phase shiftkeying or DPSK modulation types and to determine types of modulation bycomparing the histogram of an unknown signal to the known histogramtemplates stored in a histogram storage module 17. A means for histogramcomparison 18 performs automated modulation recognition by correlatingthe estimated phased histogram 16 with stored templates from thehistogram template module 17 in order to find the best match.Afterwards, the histogram comparison means 18 provides a comparisonoutput 19 to a modulation type module 20. The signal 21 can also be sentdirectly to the PSK demodulation circuit 22.

The symbol timing circuit 13 may be implemented by using an existingalgorithm or the circuit depicted in FIG. 2. Referring now to FIG. 2,there is illustrated an example of the symbol timing circuit 13. Thenormalized digitized IF signal f(k) 11 is converted to give an absolutefunction value 31 that is fed through a narrow band-pass filter 32 andphase lock loop 33 to obtain a sinusoidal signal g(k) 34 according tothis expression:

$\begin{matrix}{{g(k)} \approx {A_{f}{{\sin\left( {2\pi\frac{{kf}_{b} + d_{f}}{v_{s}}} \right)}.}}} & {{Equation}\mspace{14mu}(4)}\end{matrix}$where A_(f) is the amplitude of sinusoidal signal g(k) 34, and d_(f) isthe order of the digital band-pass FIR filter 32. The sinusoidal signalg(k) 34 is shifted by a means for DC bias, b_(DC), 35 which could be anadder, and a relay nonlinear converter 36, which results in convertingthe digitized IF signal f(k) 11 into a square waveform output g_(s)(k)37. The square waveform output g_(s)(k) 37 is depicted in more detail inFIG. 3 and the relay nonlinear function of the relay nonlinear converter36 is depicted in greater detail in FIG. 4. The purpose of using squarewaveform is to establish starting and ending time marks for counting thezero-crossing time differences.

Referring now to FIG. 3, the square waveform output g_(s)(k) 37 isrepresented by two logic states: H and L, depending upon the polarity ofthe sinusoidal waveform g(k) 11. The average number of zero-crossingpoints over L_(s) number of symbols is defined, as follows:

$\begin{matrix}{k_{cb} = {\frac{1}{L_{s}}{\sum\limits_{m = 1}^{L_{s}}{k_{cb}(m)}}}} & {{Equation}\mspace{14mu}(5)}\end{matrix}$where k_(cb)(m) is the number of zero-crossing points in the m^(th)symbol and the value of k_(cb)(m) does not need to be known inimplementation. The DC bias means 35 is given by the expression

${b_{DC} = {A_{fa}{\sin\left( {\pi\frac{k_{cb} - N_{a}}{k_{cb}}} \right)}}},$which is a constant number used to manipulate the shape of g_(s)(k), andA_(fa) is the time average of A_(f), N_(a)=[K_(cb)]−2β is an integer,

$\beta < \frac{\left\lfloor K_{cb} \right\rfloor}{2}$is a positive integer which is determined by user to eliminate the pulseshape effect, and operator └x┘ represents rounding the element x to thenearest integers towards minus infinity. Since the pulse shape isusually unknown, the expression

$\beta = {\min\left( {2,\left\lfloor \frac{\left\lfloor K_{cb} \right\rfloor - 1}{2} \right\rfloor} \right)}$will be used as the default value. The times of the rising edges ofg_(s)(k) are represented by a time sequence r(m) and the falling edgesof g_(s)(k) are represented by a time sequence d(m), m=1, 2, . . . , asshown in FIG. 3.

In FIG. 3, r(m−1), r(m) and r(m+1) are time indices of rising edges ofthe signal and d(m−1), d(m) and d(m+1) are time indices of the signal'sfalling edges. FIG. 4 depicts a function of the'relay nonlinearconverter 36. The logic state H will be given if the input value of theconverter 36 is larger than or equal to zero and the logic state L willbe given if the input value is less than zero.

Referring back to FIG. 1, the differential phase estimator 14 isimplemented by calculating the time difference between two zero-crossingsamples where the center frequency f_(c) is unknown. The IF signal f(k)11 is converted to a rectangular waveform f_(s)(k) with a time delay ofδ=sin⁻¹α by using a hysteresis nonlinear converter 51 as is shown inFIG'S 5 and 6. The threshold α=√{square root over (N₀)} can be chosenbased on the noise power N₀. A time delay d_(a) is inserted into thesignal path so that the zero-crossing points of the timing waveform g(k)will be synchronized with the edges of the symbol pulses of the signalf_(s)(k). The desired time delay is calculated according to the formula:

$\begin{matrix}{d_{a} = \left\{ \begin{matrix}d_{fs} & {if} & {d_{f} \geq d_{s}} \\{{{round}\left( {v_{s}/f_{d}} \right)} - d_{fs}} & {if} & {d_{f} < d_{s}}\end{matrix} \right.} & {{Equation}\mspace{14mu}(6)}\end{matrix}$where

$\begin{matrix}{d_{fs} = {{rem}\left( {\left( {d_{f} - d_{s}} \right),{{round}\left( \frac{v_{s}}{2f_{b}} \right)}} \right)}} & {{Equation}\mspace{14mu}(7)}\end{matrix}$and where d_(s)=└δf_(s)┘ is the number of samples that are delayed inthe signal path. The operator round(x) means to round x to the nearestinteger and the operation of z=rem(y, x) gives the remainder z after thedivision of y/x. When x(m, 1), x(m, 2) . . . , x(m, N) are N_(m) numberof zero-crossing points within the m^(th) symbol time-period andg_(s)(k)=H, then the m^(th) differential phase can be estimatedaccording to the expression:

$\begin{matrix}{{y(m)} = {2{\pi \cdot k_{cb}}{f_{b} \cdot {\sum\limits_{n = 1}^{N_{m}}{{w(n)}\left\lbrack {{x\left( {m,n} \right)} - {x\left( {{m - 1},n} \right)}} \right\rbrack}}}}} & {{Equation}\mspace{14mu}(8)}\end{matrix}$where w(n) is the weight determined by the pulse shape and

${\sum\limits_{n = 1}^{N_{m}}{w(n)}} = 1.$In general, the samples near the center of the pulse are less affectedby pulse shape so that they receive a larger weight. The weights aredetermined by the user and the default value w(n)=1/N_(m) will be usedfor all n. Defining u(m, n)=f_(s)[x(m, n)−x(m−1, n)] as the number ofzero-crossing points between x(m) and x(m−1), and N_(m) as the number ofzero-crossing points between r(m) and d(m) results in changing Equation5 to read as follows:

$\begin{matrix}{{y(m)} = {{\frac{2{\pi \cdot k_{cb}}f_{b}}{v_{s}} \cdot {\sum\limits_{n = 1}^{N_{m}}{{w(n)}{u\left( {m,n} \right)}}}} = {K_{y}{\sum\limits_{n = 1}^{N_{m}}{{w(n)}{u\left( {m,n} \right)}}}}}} & {{Equation}\mspace{14mu}(9)}\end{matrix}$where the expression:

$\begin{matrix}{K_{y} = \frac{2{\pi \cdot k_{cb}}f_{b}}{v_{s}}} & {{Equation}\mspace{14mu}(10)}\end{matrix}$is the normalization factor and is a fixed value in the given timeframe. The normalized differential phase is described as follows:

$\begin{matrix}{{z(m)} = {\frac{y(m)}{K_{y}} = {\sum\limits_{n = 1}^{N_{m}}{{w(n)}{u\left( {m,n} \right)}}}}} & {{Equation}\mspace{14mu}(11)}\end{matrix}$FIG. 5 also illustrates the function of the hysteresis nonlinearconverter 51 that is depicted in the FIG. 6 hardware implementation.FIG. 5 illustrates the point that while the input increases, if it islarger than or equal to α, then logic state H will be obtained, and ifthe input is less than α, then logic state L will be obtained.Additionally, while the input signal is decreasing, if the input is lessthan or equal to −α, the logic state L will be obtained and if it islarger than −α, the logic H will be obtained.

FIG. 6 depicts this invention's automated demodulation classificationdevice, and illustrates in more detail the operation of the differentialphase estimation module 14 of the basic FIG. 1 automated demodulationestimator 10. The computation of zero-crossing points K_(cb) in Equation9 can be performed by the zero-crossing counter 55, as further depictedin FIG'S 7 and 8. Those skilled in the art will readily see thataccuracy of symbol timing synchronization is not required for estimatingthe average number of zero-crossing points per symbol K_(cb) because theaverage value over N_(f) symbols are used. In the interests of a simplerdesign than complex prior art equipment, a constant number N≦min(N_(m))is chosen as the number of parallel paths for the diagram, which allowssimplifying Equation 11, as follows:

$\begin{matrix}{{z(m)} = {\sum\limits_{n = 1}^{N_{m}}{{w(n)}{s\left( {m,n} \right)}}}} & {{Equation}\mspace{14mu}(12)}\end{matrix}$Referring now to FIG. 6, the digitized RF signal f(k) 11 from the FIG. 1conceptual block diagram 10 is converted to rectangular waveformf_(s)(k) 52 by the hysteresis nonlinear converter 14 with the logicstate H or L being sent to a group of N number of trigger logic circuits53. FIG. 9 illustrates inputs a₁, a₂, and a₃ and output b of a triggerlogic circuit 53. The inputs and outputs of the trigger logic circuit 53are either in the logic state H or L.

Starting with the trigger logic circuit 53 and resetting all the triggerlogic outputs to L, when both the rectangular waveform f_(s)(k) 52 anddigitized RF signal g_(s)(k) 51 are in logic state H, the trigger signal54 of trigger logic circuit 53 will be logic state H and the output ofthe trigger logic circuit 53 will be b=H and will feed back to the inputa₃ to lock the signal 54 of trigger logic circuit 53 to H. When bothrectangular waveform f_(s)(k) 52 and digitized RF signal g_(s)(k) 51 arein logic state L, the signal 54 will be logic state L and the output b=Lwill feed back to the input a₃ to lock the signal 54 to L, which resultsin the trigger signal 54 being sent to a number of zero-crossingcounters 55.

In operation, the zero-crossing counter 55 is reset to zero by theraising edge of the trigger signal 54, logic state H, and starts tocount the number of clock pulses s(m, 1). When the output of thezero-crossing counter 55 is larger than zero, the zero-crossing counter55 “Hit” port of FIG. 8 outputs a logic state H to enable an additionaltrigger logic circuit 53 and also count the number of zero-crossings s(m, 2). Therefore, the number of zero-crossings u(m,n), n=1, 2, . . . ,N, is counted in order, shifted, weighted, and added to form thenormalized differential phase output z(m) 15.

FIG. 8 depicts the symbol of the zero-crossing counters 55 as havingfour ports: Reset, Clock, Count and Hit. The output is held to its mostrecent value between triggering events and will reset the zero-crossingcounter 55 to its initial state when the trigger event is received atthe Reset input. When trigger events are received simultaneously at the“Clock” and “Reset” ports, the zero-crossing counter 54 is first reset,and then increases or decreases, as appropriate. The “Count” portproduces the current value of the zero-crossing counter 54 as asample-based scalar with the same sample period as the inputs. The “Hit”port produces zeroes while the value of the counter value is equal tozero and produces one when the counter value is larger than zero.

Referring back to FIG. 6, the histogram of the normalized differentialphased outputs z=(m), m=1, 2, . . . , L, can be calculated usingstandard statistical approaches. M histogram peaks will be observed forM-PSK and M-DPSK modulated signals and one peak will be observed for acarrier. Those peaks are used for recognizing different modulationtypes. The histogram templates from the FIG. 1 histogram storage module17 are used for automatic modulation recognition by correlatingnormalized differential phases to known templates. The histogramtemplates may be formed with the current method described by Hsue andSoliman and should then be normalized using the factor K_(y). It isnoted that if an automated histogram normalization procedure is used,the calculation of K_(y) can be omitted. The histogram of the unknownsignal will be correlated to N_(t) number of known templates to resultin N_(t) number of correlation scores. Since each score is associatedwith a known modulation type, the highest score indicates a recognitionresult. A threshold is chosen to exclude the low confidence results. Ifa score is below the threshold, the classification is reported asfailure.

FIG. 7 is a block diagram for estimating the average number ofzero-crossings using the zero-crossing counter 55 of the presentinvention. A sinusoidal symbol timing signal g(k) signal 53 from theFIG. 2 phase loop lock 33 is used to reset the zero-crossing counter 55.The rectangular waveform f_(s)(k) 52 is also sent to the zero-crossingcounter 55 in order to provide the zero-crossing input. A zero-crossingcount output 61 is sent to a group of shift registers 62 and adders 35in order to count and average the number of zero crossing points persymbol k_(cb) output 64, which is used by Equation 10. It should benoted that if an automated histogram normalization procedure is used,the calculation of k_(cb) is not needed.

FIG. 9 depicts the symbols for the trigger logic circuit 52. The logicrelation between input and output is defined in Truth Table 1.

The present invention also includes a software embodiment for automaticdemodulation and classification of phase shift keying signals withhysteretic differential zero-crossing time samples. FIG. 10 is a flowchart depicting the operation of this software embodiment. Referring nowto FIG. 10, for a given N, there is depicted a means for inputting knownvariables, represented by box 41, where values for the L_(s), weightw(N), and symbol frequency f_(b) are given as known inputs. A firstsoftware calculating module, represented by Box 42, calculates theaverage number of zero-crossing points per symbol k_(cb). Notice that ifan automated histogram normalization procedure is used, the calculationof k_(cb), is not needed. Box 43 represents a means for generating atiming signal g(k), which is then used in an index determining module,represented by Box 44 to determine timing indices r(m) and d(m).Phase-shifting classification occurs in a means for phase-shiftingclassification, Box 45. L_(s)−1 number of symbols is used incalculations which are regulated by an index m. In those cases where mequals L_(s), the zero-crossing calculation is completed and thecalculation result is delivered for PSK/DPSK classification with PSKdenoting phase-shift keying and DPSK denoting differential phase-shiftkeying by a means for differential phase-shift keying. However, in thosecases where m is less than L_(s), but the time variable t(k) in block 47is beyond the indices r(m) and d(m), the index m increases by one unit.Otherwise, if the inequality condition in block 47 is satisfied, acounting module, represented by box 46, counts the number ofzero-crossings N and the number of zero-crossing times x(m, 1), x(m, 2),. . . , x(m,N). In a saving module, represented by box 48, the lastzero-crossing times x(m−1, 1), x(m−1, 2), . . . , x(m−1, N) are savedfor the further computation. A differential phase y(m) is calculated ina second calculating module, represented by Box 49A, and value z(m) iscalculated in a third calculating module, represented by Box 49B, basedon a number of symbols L_(s), weight w(N), symbol frequency f_(b), andthe last and current zero-crossing times counted in the saving steprepresented by Box 48, which results in a DPSK demodulation output.

FIG. 11 is a chart illustrating an example of a IF signal with QPSKmodulation scheme where the solid-line stands for the IF signal waveformf(k), the dashed-line stands for the square waveform g_(s)(k) used forthe symbol timing signal to indicate the starting and ending points incounting the zero-crossing points, the circles stand for the zerocrossing points and squares stand for the approximate starting points ofsymbols. The zero-crossings near the symbol transition time periods areignored in differential phase estimation since they are noisy and notreliable. A field collected QPSK signal was tested for demonstrationpurposes. The original signal had a sampling frequency of 46,387.33 Hz,a symbol frequency of 11,598 Hz, a center frequency offset of 652 Hz, aroot-raised cosine pulse shaping (α=0.35) and a signal-to-noise rationof 17 dB. In order to achieve better resolution, the signal in this casewas over-sampled to 40 samples per symbol and the center frequency isshifted to a higher rate of f_(c)=11,589 Hz in order to have the betterresolution in zero-crossing calculation. Notice that the waveform f(k)does not indicate the signal modulation scheme directly, which issimilar to various modulation schemes. The goal of counting thezero-crossing points is to conduct differential phase (which is thefeature of the modulation scheme) calculation. The result of thedifferential phase indicates and classifies the unknown modulationscheme. This chart illustrates the point that better resolution inzero-crossing calculation is achieved because of the relationshipbetween a higher center frequency, an increase in the number ofzero-crossing points, resulting in a higher resolution in calculation.The differential phase can be calculated by averaging the timedifference of the zero-crossings, x(m, n)−x(m−1, n) between symbols. Itshould be noted that the zero-crossings near the symbol transition timeperiods are ignored in differential phase estimation since they arenoisy and not reliable.

The present invention also encompasses an automated zero-crossing signalsurveillance demodulation and classification device for rapidlyidentifying an unknown modulation in a signal with many of the samevariations and embodiments as the automatic zero-crossing signaldemodulation and classification device for rapidly identifying anunknown modulation in a signal.

The present invention also contemplates methods for automaticdemodulation and classification of phase shift keying signals withhysteretic differential zero-crossing time samples. The steps of thisinvention's methods are also depicted by FIG. 10. Referring again toFIG. 10, for a given N, this invention's method commences with the stepof inputting known variables, represented by box 41, where values forthe L_(s), weight w(N), and symbol frequency f_(b) are given as knowninputs. During a first calculating step, represented by Box 42, theaverage number of zero-crossing points per symbol K_(cb) is calculatedwith a software calculating module. Notice that if an automatedhistogram normalization procedure is used, the calculation of k_(cb) isnot needed. Box 43 represents a timing signal generating step forgenerating a timing signal g(k), which is then used in the Box 44 stepof determining timing indices r(m) and d(m). This invention's methodcontinues with a phase-shifting classification step. In those caseswhere m equals L_(s), phase-shifting classification takes place, withPSK denoting phase-shift keying and DPSK denoting differentialphase-shift keying. However, in those cases where m is less than L_(s),this invention's method continues with a counting step, represented byBox 46, for counting the number of zero-crossings N and countingzero-crossing times x(m, 1), x(m, 2), . . . , x(m,N) based on the timingindices r(m) and d(m) depicted in the indexing step represented by Box47. In a saving step, represented by Box 48, the last zero-crossingtimes x(m−1, 1), x(m−1, 2), . . . , x(m−1, N) are saved for the furthercomputation. A differential phase y(m) is calculated in a secondcalculating step, represented by Box 49A, and value z(m) is calculatedin a third calculating step, represented by Box 49B, based on a numberof symbols L_(s), weight w(N), symbol frequency f_(b), and the last andcurrent zero-crossing times counted in the saving step represented byBox 48, which results in a DPSK demodulation output.

Many of the variations and embodiments of the automatic zero-crossingsignal demodulation and classification device and automatedzero-crossing signal surveillance demodulation and classification devicealso apply to this invention's methods.

It is to be further understood that other features and modifications tothe foregoing detailed description of the automatic zero-crossing signaldemodulation and classification devices and methods are within thecontemplation of the present invention, which is not limited by thisdetailed description. Those skilled in the art will readily appreciatethat any number of configurations of the present invention and numerousmodifications and combinations of materials, components, geometricalarrangements and dimensions can achieve the results described herein,without departing from the spirit and scope of this invention.Accordingly, the present invention should not be limited by theforegoing description, but only by the appended claims.

What I claim is:
 1. An automatic zero-crossing signal demodulation andclassification device for rapidly identifying an unknown modulation in asignal, comprising: a normalized digitized intermediate frequency (IF)phase-shifted keying signal f(k) is sent to a sample delay circuit and asymbol timing circuit; said IF signal having an amplitude, a pluralityof random amplitude fluctuations at multiple zero crossing points, asusceptibility to random noise, an unknown center frequency fc and anunknown modulation; said sample delay circuit produces a delayed signalfed into a differential phase estimator; said differential phaseestimator having a plurality of zero-crossing counters and a means fordifferential phase shift keying; said symbol timing circuit, having ameans for DC bias sends a symbol timing input to said differential phaseestimator and converts said IF signal to a sinusoidal signal g(k); saidsinusoidal signal g(k) is shifted to said DC bias means and is convertedinto a square waveform output gs(k); said differential phase shiftkeying means codes said symbol timing input; said differential phaseestimator calculates a time difference value between a plurality ofzero-crossing samples and converts said IF signal to a rectangularwaveform fs(k) having a time delay, said time delay synchronizes aplurality of zero-crossing points of a timing waveform g(k); saiddifferential phase shift keying means demodulates said delayed signaland said symbol timing input and said differential phase estimatorprovides a differential phase output to a means for generating anestimated phase histogram; said histogram generating means generates anestimated phased histogram and evaluates a plurality of statisticalpatterns to determine a plurality of modulation types; a means forhistogram comparison compares said estimated phased histogram to saidplurality of known templates from a histogram storage module todetermine an optimal match; and said histogram comparison means providesa comparison output to a modulation type module, said modulation typemodule rapidly develops an increased accuracy modulation output thatidentifies said unknown modulation for a user on a display means.
 2. Theautomatic zero-crossing signal demodulation and classification devicefor rapidly identifying an unknown modulation in a signal, as recited inclaim 1, said differential phase estimator further comprising ahysteresis nonlinear converter, a plurality of trigger logic circuits,an adder and an OR logic.
 3. The automatic zero-crossing signaldemodulation and classification device for rapidly identifying anunknown modulation in a signal, as recited in claim 2, said symboltiming circuit further comprising an absolute function, a band-passfilter, a phase lock loop, an adder and a relay nonlinear converter. 4.The automatic zero-crossing signal demodulation and classificationdevice for rapidly identifying an unknown modulation in a signal, asrecited in claim 3, further comprising: a hysteresis converter inputbeing sent to said hysteresis nonlinear converter; said square waveformg_(s)(k) being represented by an H logic state or an L logic statedepending upon a sinusoidal waveform polarity of said sinusoidalwaveform g(k) said hysteresis converter input being increased to aninput value greater than or equal to α, obtains said logic state H; andsaid hysteresis converter input being decreased to an input value lessthan or equal to −α, obtains said logic state L.
 5. The automaticzero-crossing signal demodulation and classification device for rapidlyidentifying an unknown modulation in a signal, as recited in claim 4,further comprising said hysteresis nonlinear converter converts saiddelayed signal into a rectangular waveform f_(s)(k) with a time delay ofδ=sin⁻¹α.
 6. The automatic zero-crossing signal demodulation andclassification device for rapidly identifying an unknown modulation in asignal, as recited in claim 5, further comprising identifying saidunknown modulation from a plurality of non-cooperative cornmunications.7. An automated zero-crossing signal surveillance demodulation andclassification device for rapidly identifying an unknown modulation in asignal, comprising: a normalized digitized intermediate frequency (IF)phase-shifted keying signal f(k) is sent to a sample delay circuit and asymbol timing circuit; said IF signal having an amplitude, a pluralityof random amplitude fluctuations at multiple zero crossing points, asusceptibility to random noise, an unknown center frequency fc and anunknown modulation; said sample delay circuit produces a delayed signalfed into a differential phase estimator; said differential phaseestimator having a plurality of zero-crossing counters and a means fordifferential phase shift keying; said symbol timing circuit, having ameans for DC bias sends a symbol timing input to said differential phaseestimator and converts said IF signal to a sinusoidal signal g(k); saidsinusoidal signal g(k) is shifted to said DC bias means and is convertedinto a square waveform output gs(k); said differential phase shiftkeying means codes said symbol timing input; said differential phaseestimator calculates a time difference value between a plurality ofzero-crossing samples and converts said IF signal to a rectangularwaveform fs(k) having a time delay, said time delay synchronizes aplurality of zero-crossing points of a timing waveform g(k); saiddifferential phase shift keying means demodulates said delayed signaland said symbol timing input and said differential phase estimatorprovides a differential phase output to a means for generating anestimated phase histogram; said histogram generating means generates anestimated phased histogram and evaluates a plurality of statisticalpatterns to determine a plurality of modulation types; a means forhistogram comparison compares said estimated phased histogram to saidplurality of known templates from a histogram storage module todetermine an optimal match; and said histogram comparison means providesa comparison output to a modulation type module, said modulation typesmodule rapidly develops an increased accuracy modulation output thatidentifies said unknown modulation for a user on a display means.
 8. Theautomated zero-crossing signal surveillance demodulation andclassification device for rapidly identifying an unknown modulation in asignal, as recited in claim 7, said differential phase estimator furthercomprising a hysteresis nonlinear converter, a plurality of triggerlogic circuits, an adder and an OR logic.
 9. The automated zero-crossingsignal surveillance demodulation and classification device for rapidlyidentifying an unknown modulation in a signal, as recited in claim 8,said symbol timing circuit further comprising an absolute function, aband-pass filter, a phase lock loop, an adder and a relay nonlinearconverter.
 10. The automated zero-crossing signal surveillancedemodulation and classification device for rapidly identifying anunknown modulation in a signal, as recited in claim 9, furthercomprising: a hysteresis converter input being sent to said hysteresisnonlinear converter; said square waveform g_(s)(k) being represented byan H logic state or an L logic state depending upon a sinusoidalwaveform polarity of said sinusoidal waveform g(k) said hysteresisconverter input being increased to an input value greater than or equalto α, obtains said logic state H; and said hysteresis converter inputbeing decreased to an input value less than or equal to −α, obtains saidlogic state L.
 11. The automated zero-crossing signal surveillancedemodulation and classification device for rapidly identifying anunknown modulation in a signal, as recited in claim 2, furthercomprising said hysteresis nonlinear converter converts said delayedsignal into a rectangular waveform f_(s)(k) with a time delay ofδ=sin⁻¹α.
 12. The automated zero-crossing signal surveillancedemodulation and classification device for rapidly identifying anunknown modulation in a signal, as recited in claim 11, furthercomprising identifying said unknown modulation from a plurality ofnon-cooperative communications.
 13. A method for automatic zero-crossingdemodulation and classification of an unknown modulation signal,comprising the steps of: inputting a plurality of known variables;forming a sample delay circuit; forming a symbol timing circuit;configuring a differential phase estimator; sending a normalizeddigitized intermediate frequency (IF) phase-shifted keying signal f(k)to said sample delay circuit and said symbol timing circuit, said IFsignal having an amplitude, a plurality of random amplitude fluctuationsat multiple zero crossing points, a susceptibility to random noise, anunknown center frequency fc and an unknown modulation; converting saidIF signal into a delayed signal with said sample delay circuit; feedingsaid delayed signal fed into said differential phase estimator, saiddifferential phase estimator having a plurality of zero-crossingcounters and a means for differential phase shift keying; sending asymbol timing input to said differential phase estimator from saidsymbol timing circuit, said symbol timing circuit, having a means for DCbias sends a symbol timing input to said differential phase estimatorand converts said IF signal to a sinusoidal signal g(k); calculating anaverage number of zero-crossing points; coding said symbol timing inputwith a differential phase shift keying means; shifting said sinusoidalsignal g(k) to said DC bias means and converting said sinusoidal signalg(k) into a square waveform output gs(k); calculating a time differencevalue between a plurality of zero-crossing samples with saiddifferential phase estimator and converting said IF signal to arectangular waveform fs(k) having a time delay, said time delaysynchronizes a plurality of zero-crossing points to generate a timingwaveform g(k); calculating a value z(m) to provide a differential phaseoutput to a means for generating an estimated phase histogram by saiddifferential phase shift keying means demodulating said delayed signaland said symbol timing input; generating an estimated phased histogramin said histogram generating means; evaluating a plurality ofstatistical patterns to determine a plurality of modulation types saidhistogram generating means; retrieving a plurality of known templatesfrom a histogram storage module; comparing said estimated phasedhistogram to said plurality of known templates; correlating saidestimated phased histogram and said stored plurality of histogramtemplates with a means for histogram comparison; determining a bestmatch histogram template; generating a comparison output to a modulationtype module in said histogram comparison means; and rapidly developingan increased accuracy modulation output that identifies said unknownmodulation for a user on a display means.
 14. The method for automaticzero-crossing demodulation and classification of an unknown modulationsignal, as recited in claim 13, further comprising the step ofconfiguring said differential phase estimator to include a hysteresisnonlinear converter, a plurality of trigger logic circuits, an adder andan OR logic.
 15. The method for automatic zero-crossing demodulation andclassification of an unknown modulation signal, as recited in claim 14,further comprising the step of forming said symbol timing circuit withan absolute function, a band-pass filter, a phase lock loop, an adderand a relay nonlinear converter.
 16. The method for automaticzero-crossing demodulation and classification of an unknown modulationsignal, as recited in claim 15, further comprising the steps of: sendinga hysteresis converter input to said hysteresis nonlinear converter; andrepresenting said square waveform g_(s)(k) with an H logic state or an Llogic state depending upon a sinusoidal waveform polarity of saidsinusoidal waveform g(k) said hysteresis converter input being increasedto an input value greater than or equal to α, to obtain said logic stateH and said hysteresis converter input being decreased to an input valueless than or equal to −α, obtain said logic state L.
 17. The method forautomatic zero-crossing demodulation and classification of an unknownmodulation signal, as recited in claim 16, further comprising the stepof converting said delayed signal f(k) into a rectangular waveformf_(s)(k) with a time delay of δ=sin⁻¹α with said hysteresis nonlinearconverter.
 18. The method for automatic zero-crossing demodulation andclassification of an unknown modulation signal, as recited as recited inclaim 17, further comprising the step of identifying said unknownmodulation from a plurality of non-cooperative communications.